No. 4.—About Jack Frost.
BY W. J. ROLFE, A.M.
With the single exception of water, all substances expand, or become larger, when heated, and contract, or become smaller, when cooled. This is seen in metals better than in most other bodies. An iron ball which when cold will just pass through a certain ring will not do so after being put in boiling water. The tires of carriage wheels before being put on are heated in a fire, in order that their contraction in cooling may make them bind more tightly. On a railroad a little space is left between the ends of the rails to allow them to expand. If this were not done, their lengthening in hot weather would bend them outward or inward, so that they would not be exactly parallel, and this might be the cause of serious accidents. It has been proved that Bunker Hill Monument—a granite pile 220 feet high—is bent to one side by the expanding of the opposite side when the sun shines upon it; and similar changes must take place in every tower, or steeple, or other tall structure exposed to the sun's rays.
The least change in the temperature of any material produces a change in its size, though not in its weight; and if one part is heated or cooled more than another, the shape of the whole must be somewhat altered.
Water contracts until it is cooled down to 40 degrees Fahrenheit—that is, 40 degrees of our common thermometers, or 8 degrees above the freezing-point—and then it expands until it freezes. This is a wise provision of nature. If water kept on contracting with cold, it would begin to freeze at the bottom, where the coldest portions of it would settle by their weight, and this would go on until it was all frozen, so that in winter our lakes and rivers would become solid masses of ice. This would kill all fishes and other animals in the water, and all the heat of summer would not suffice to liquefy these great bodies of ice. As it is, the water begins to freeze at the surface, and the layer of ice keeps the water below it from freezing; for though the ice is itself cold, a wall of it will keep out the cold as well as a wall of stone or brick.
The force with which water expands in freezing is almost irresistible. The freezing of half a gill of water in a confined space will lift a weight of several tons. A thick iron bomb-shell filled with water will be split open by the freezing of the liquid as it would be by a charge of gunpowder. In winter the water-pipes in our houses are often burst by the freezing of their contents. In some parts of England advantage is taken of this property of water in the slate quarries. Large blocks of slate are placed where the rain will fall upon their edges. The water works its way between the layers, freezes, and splits the mass into thin plates.
Jack Frost has done a good deal of this rock-splitting on his own account. A large part of the soil of our world has been made by the freezing of water in the cracks and crevices of rocks. Mountains have thus been rent asunder and pulverized, and the work goes on every winter. And after the soil has been formed, it is broken up and crumbled by the action of frosts and thaws. Jack Frost is a good helper to the ploughman and the farmer.
He works also on a grander scale than this. In many parts of the earth, as you know, there are great rivers of ice called glaciers. They not only look like rivers, but they flow like them, though so slowly that we can not see the motion. In the course of a year they move only a few hundred feet, but with mighty force, grinding the sides and bottom of the valley as they go, breaking off huge masses of rock, and bearing them along, together with smaller stones, earth, and mud. Thus they are gradually tearing down the hills and filling up the valleys.
Ages ago vast glaciers swept in this way over a large portion of the Northern hemisphere, and in many places we can see how they ground and scratched the sides of mountains and surfaces of rocks on their way. The big stones known as "bowlders" that abound in many parts of the country were brought and dropped by these moving masses of ice, and in some cases we can tell just where they came from, perhaps hundreds of miles away.